This is the first application filed for the present invention.
Not applicable.
The present invention relates in general to the preparation of subterranean wells for the production of fluids from underground reservoirs and, in particular, to tools used in subterranean wells for casing perforation and production stimulation.
As the supply of highly-productive hydrocarbon wells is exhausted, there is increasing interest in producing hydrocarbon fluids from potentially productive geological formations that contain sufficient volume of such fluids, but have low permeability so that production is slow or difficult. In order to economically produce fluids from such formations, the formations must be artificially xe2x80x9cstimulatedxe2x80x9d to increase the permeability of the production zone. Many methods have been invented for artificially stimulating subterranean formations. Generally, such methods are referred to as xe2x80x9cwell fracturingxe2x80x9d. During well fracturing, pressurized fluids are pumped through perforations in a well casing and into a production zone in order to break or fracture pores in the zone to improve permeability so that the hydrocarbon fluids can drain from the production zone into the casing. Those pressurized fluids are often laden with abrasive xe2x80x9cproppantsxe2x80x9d, such as sharp sand. In order to stimulate a new well, it is first necessary to perforate a casing of the well. This is generally accomplished using what is known in the art as xe2x80x9cperforating gunsxe2x80x9d such as taught, for example, in U.S. Pat. No. 4,598,776 which issued on Jul. 8, 1986 to Stout. After the casing is perforated, a fracturing tool is lowered into the well and fluids pressurized to 5,000-10,000 psi are pumped through the perforations into the formation. The high pressures tend to break up the formations to release trapped hydrocarbon fluids. The proppants infiltrate the formation and prevent collapse after the high stimulation pressure is released.
In order to increase the efficacy of the stimulation process, xe2x80x9cstaged well stimulationxe2x80x9d methods have been developed. In staged stimulation, small sections of a production zone are fractured in sequence by isolating sections of the production zone or, if the production zone is very small, isolating the entire production zone in order to concentrate the area to which stimulation fluids are delivered. This helps ensure that a production zone is more evenly fractured. It is common practice today to perforate all of the production zones through which a casing extends. Tubing is then run into the well with isolation packers to isolate a section of a zone to be stimulated. Generally, about 10 feet (3.3 meters) of a zone is isolated at a time using isolation packers and a small fracturing treatment is applied to that section of the zone. Thereafter, the tubing is moved up and another small fracturing treatment is performed. This process is repeated until all of the production zones in the well have been stimulated.
A disadvantage of the way in which the staged stimulation process is performed is that if a condition known as xe2x80x9cscreenoutxe2x80x9d occurs, the entire toolstring must be pulled from the well in order to clean the apparatus and recommence the stimulation. Screenout is a phenomenon that occurs when abrasive proppants clog the injection tubing during a fracturing process.
Furthermore, if all of the production zone(s) are already perforated, and there are open perforations above the upper isolation packer, fracturing fluids may migrate upwards through a production zone outside the casing and enter the casing above the upper isolation packer. This can cause the casing to fill with high pressure fluid and proppants. This has two disadvantages. First, the casing above the upper isolation packer may fill with abrasive proppant and trap the tool within the casing. Second, the fracturing of one or more sections may be ineffective because the fracturing fluids follow a path of least resistance and the entire production zone is not uniformly fractured.
Some of these problems with prior art methods of staged well stimulation are overcome by inventions described in U.S. Pat. Nos. 5,865,252 and 6,116,343 which issued respectively on Feb. 2, 1999 and Sep. 12, 2000 to Van Petegem et al. These patents describe a method and apparatus for a one-trip production zone perforation and proppant fracturing operation carried out using a workstring-supported perforation gun lowered into a casing nipple located in the production zone. Firing of the perforation gun creates spaced apart aligned sets of perforations extending outwardly through a side wall portion of the workstring, the nipple, cement surrounding the nipple and into the production zone. After firing, the gun falls into and is retained in an underlying gun catcher portion of the workstring. Proppant slurry is then pumped down the workstring and out through the side wall perforations, and through the aligned perforations in the nipple to stimulate the production zone. After stimulation of the production zone, the workstring and the spent perforation gun that it retains are pulled up out of the casing.
While this method represents an advance over the prior art, it still has several disadvantages. First, the purpose of the invention is to enable a one-trip entry run into the well to perform perforation as well as stimulation. In order to make the one-trip into the well profitable, a significant length of the nipple must be perforated and stimulated in a single shot. Consequently, if the perforated area is very large, the production zone may not be evenly stimulated. In other words, this tool is not suited for economical staged stimulation.
Furthermore, the perforating gun is designed for single-shot operation. As described above, after the perforating gun is fired, it is dropped into a gun-catching section of the toolstring. Because of this, only one contiguous region of a casing can be perforated each time the tool is run into the well. Consequently, the tool must be run into the well at least once for each production zone requiring stimulation. This is time-consuming and contributes to the cost of production from the well. A further disadvantage is the fact that a perforating gun must be customized for each production zone. While this is commonplace for wireline applications, it is more difficult and time-consuming when the perforating gun must be incorporated into a toolstring between well perforation/stimulation operations.
There therefore exists a need for a method and apparatus that permits selective perforation and stimulation of staged sections of a production zone while ensuring an even and complete distribution of fracturing fluids within each stage of the production zone treated.
There is also a need for a method and apparatus that permits a plurality of production zones, or stages in a production zone to be successively perforated and stimulated without withdrawing a toolstring from the well.
It is therefore an object of the invention to provide a method and apparatus for conducting subterranean well casing perforation and production zone stimulation using a process that saves time and reduces costs.
It is a further object of the invention is to provide a well tool for perforating and stimulating subterranean wells so that both a staged perforating process and a staged stimulation process can be completed using the well tool in a one-trip insertion of the tool into the well.
It is another object of the invention to provide a well tool for selectively perforating and stimulating subterranean wells so that the perforation of several selected sections of a well casing and the stimulation of several corresponding sections of one or more subterranean production zones can be completed using the well tool in a one-trip insertion of the tool into the well.
It is a further object of the invention to provide a method for completing perforation and stimulation of selected sections of a subterranean well using a well tool adapted to perform staged perforation and fracturing in a one-trip insertion of the tool into the well.
In general terms, these objects are achieved by pumping production stimulation fluid through a novel perforating gun to an injection nozzle connected to a lower end of the gun.
In accordance with one aspect of the invention, an apparatus is provided for perforating and stimulating subterranean wells, such as oil wells, which comprises a perforating gun for perforating a well casing, the perforating gun being adapted for connection of a tubing string and including a fluid passage to permit well stimulation fluid to be pumped therethrough without affecting functionality of components of the gun; and an injection nozzle connected to a second end of the perforating gun in fluid communication with the fluid passage for injecting pressurized stimulation fluid into a production zone through perforations made through a casing of the well by the perforating gun.
The charges of the perforating gun are preferably adapted to be selectively detonated so that perforations in selected sections of the well casing can be effected by a single perforating gun. The injection nozzle preferably includes an annular seal attached to a bottom end of the nozzle for sealing an annulus between the apparatus and the well casing to prevent pressurized stimulation fluid from flowing into the well casing below the apparatus. In order to balance fluid pressure on each side of the annular seal when the apparatus is inserted into the well, it is preferable to provide a fluid passage having a check valve in the injection nozzle to permit fluid in the well below the annular seal to flow through the injection nozzle when the apparatus is inserted into the well, while inhibiting the pressurized stimulation fluid from flowing through the nozzle into the well casing below the apparatus.
In accordance with one embodiment of the invention, the perforating gun includes a tubular sleeve and an inner mandrel which define an annulus to accommodate charges carried by the perforating gun. The injection nozzle includes a sidewall, an axial passage in fluid communication with the inner mandrel of the perforating gun, and a plurality of radial passages for directing pressurized well stimulation fluid into an annulus between the apparatus and the well casing above the annular seal. A check valve is located in the axial passage below the radial passages. The apparatus preferably includes a collar locator for detecting joint collars between sections of the well casing to permit a position of the apparatus to be tracked as the apparatus is moved up or down through the well casing. The apparatus also preferably includes a temperature sensor and a pressure sensor for measuring the downhole temperature and pressure during a well stimulation process.
In accordance with another aspect of the invention, there is provided a method for perforating and stimulating subterranean cased wells, comprising steps of perforating a selected section of the cased well by selectively firing charges from a perforating gun; and stimulating the selected section by pumping stimulation fluid down a fluid passage through the perforating gun and through perforations made by the selectively fired charges.
In accordance with yet another aspect of the invention, there is provided a method for perforating and stimulating subterranean cased wells, comprising steps of inserting a tool into a well, the tool including a perforating gun that carries vertically spaced apart charges that may be selectively fired for perforating the well casing, the perforating gun being adapted for connection to a tubing string and including a fluid passage to permit well stimulation fluid to be pumped therethrough, and an injection nozzle connected to an opposite end of the perforating gun in fluid communication with the fluid passage; positioning the tool in the well so that a selected number of the charges carried by the perforating gun are positioned within a selected section of a production zone to be stimulated; detonating the selected number of charges to perforate the well casing; pulling the tool upward until the injection nozzle is positioned in the selected section of the production zone; and pumping the well stimulation fluid through the tubing string, the fluid passage and the nozzle so that the well stimulation fluid is injected through the perforations in the well casing into the selected section of the production zone.
When more than one section of a cased well is to be perforated and stimulated, the above steps a)-e) are begun at a lowest section of the cased well and the well is treated in a staged upward progression. During each stage of the well treatment, the tool is first positioned to locate a selected group of charges in an area of the casing to be perforated. The selected group of charges are detonated to perforate the well casing and steps d) and e) are repeated until the stimulation of a production zone is completed. Steps b) to e) are then repeated for other production zones of the well. Remaining unused charges are selectively detonated until the entire perforation and stimulation of all selected sections of the well are completed, or all of the charges are spent. Finally, the tool is removed from the well.
The apparatus and method in accordance with the invention provide an improved solution for well completion. Perforation and stimulation of more than one section of the subterranean well is enabled without withdrawing and reinserting the toolstring between stimulation sessions for various sections of the well. As a result, the time required to complete a well is significantly reduced, and costs are correspondingly reduced.
Other features and advantages of the invention will be better understood with reference to preferred embodiments described below.